A technique by which a transistor is formed using a semiconductor thin film formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor. As another material for the same, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration lower than 1018/cm3 is disclosed (see Patent Document 1).
Although a transistor including an oxide semiconductor can be operated at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon, the transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of a change in electrical characteristics. For example, the threshold voltage of the transistor might be changed after a bias-temperature stress test (BT test). Note that in this specification, threshold voltage refers to a gate voltage which is needed to turn on a transistor. A gate voltage refers to a potential difference between a source potential and a gate potential when the source potential is regarded as a reference potential.